Regenerated cellulose sheets and process of producing the sheets



y 8. 1956 P. H. SCHLOSSER ETAL 2,744,292

REIGENBRATED CELLULOSE SHEETS AND PROCESS OF PRODUCING THE SHEETS Filed Feb. 10, 1953 1 VE TORS P 6c 10: e lfi'fa'layan Mf /7;

United States Patent REGENERATED CELLULOSE SHEETS AND PROCESQ OF PRODUCING THE SHEETS Paul H. Schlosser and Reid Logan Mitchell, Shelton, Wasln, assignors to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware Application February 10, 1953, Serial No. 336,136

11 Claims. (Cl. 18-48) This invention relates to regenerated cellulose and has for its object the provision of an improved process for producing regenerated cellulose sheets, and improved sheets formed of regenerated cellulose and having residual portions of cellulose fibers dispersed therein. The process of our invention is concerned with, or is a modification of, the viscose process in that it involves the production of alkali cellulose, its xanthation, and its regeneration to cellulose. However, our process provides improvements in the production of sheets by the viscose process characterized primarily by the direct conversion of cellulose xanthate while in sheet form to regenerated cellulose without intermediate solution and extrusion operations.

In the conventional production of regenerated cellulosic sheet material by the viscose process, fibrous chemical cellulose is steeped with caustic soda liquor to form alkali cellulose, pressed to the desired ratio of liquor to pulp, shredded to a fine crumb, stored in cans to depolymcrize, treated with carbon bisulfide to form cellulose xanthate, dissolved in dilute caustic soda solution to form viscose, filtered to remove undissolved fiber fragments, ripened to a given content of xanthate sulfur, extruded through a thin slot into an acid casting bath to coagulate and regenerate the cellulose into a continuous film of cellulose. in such operations the fibrous cellulose raw material in sheet form is reduced in several discontinuous operations to small crumbs, dissolved, filtered, and then precipitated again in sheet form.

It is the principal object of our invention to perform the basic reactions of the viscose process, but in a continuous and much simplified operation to produce at much reduced cost improved regenerated sheet material of various types which are especially suitable for various uses.

In accordance with the process of our invention we form a cellulose xanthate sheet and, without resorting to the usual step of dissolving the xanthate to form a viscose solution, we regenerate the solid cellulose xanthate sheet directly. Not only do we omit the dissolving step and the complications necessary to handle a viscous liquid, the filtering and the difficulties it entails, and the entire extrusion stage, but we retain in the sheet residual portions of the cellulose fibers which reentorce, strengthen, and toughen the sheet. While we may regenerate the solid concentrated cellulose Xanthate sheet, we may also effect a partial solution of the sheet, for example, one surface may be partially dissolved by spraying it with water or dilute caustic. The viscose solution associatcd with the solid cellulose xanthate of the sheet contains much more cellulose than the conventional viscose solutions which have concentrations of but from 7% to 9% cellulose. We may vary the extent to which a portion of the sheet is diluted and dissolved depending upon the properties desired in the regenerated sheet.

In one of the more complete and preferred embodiments of the invention, we compress the porous cellulose xanthate, either when entirely in fibrous concentrated form or when the fibrous concentrated xanthate is asso- 2,744,292 Patented May 8, 1956 ciated with a small amount of diluting solution as above described, into a dense sheet as in a calendering operation, and then treat the dense sheet directly with an acid regenerating solution. When forming a relatively transparent sheet resembling cellophane, we prefer to apply to the fibrous cellulose xanthate just prior to pressing in the calendering operation, a small amount of dilute sodium hydroxide solution to prepare in efiect a cellulose xanthate solution in sheet form having a relatively high concentration of cellulose, for example from 20 to 35% of cellulose. At this time we may also add to the xanthate sheet a wetting agent, a plasticizer, a softening agent, a regenerating assistant, or a water repelling agent to impart to the regenerated sheet the characteristics resulting from these agents.

In its more complete embodiment, our invention provides a continuous sheet-treating and sheet-forming process in which sheets of cellulose such as high purity dissolving-type chemical cellulose, paper of various degrees of purity such as unbleached tissue or mechanic cloth, and cotton webs or fabrics such as cotton batting or cheese-cloth are successively treated with caustic soda to form alkali cellulose, with carbon bisulfide to form cellulose xanthate, if desired, the addition of a solvent such as water or dilute caustic soda solution to dissolve a part of the fibrous xanthate sheet, compressed as in a calendering operation to form a thin, dense continuous sheet of cellulose xanthate, and with an aqueous acid regenerating solution to form a regenerated cellulose sheet having therein the residual partially reacted portions of cellulose fibers or residual fibrous skins (hereinafter called residual portions of cellulose fibers). We may take a sheet of cellulose pulp, for example, supplied in the form of a large roll of indefinite length, and pass the sheet through a steeping operation to efi'ect a highly uniform conversion of the pulp to alkali cellulose, advantageously in pressed sheet form, as described in our United States Patent 2,614,102. This sheet is passed continuously through a xanthating operation, preferably by treating the alkali cellulose sheet with carbon bisulfide according to the process of our copending application, Serial No. 274,208, filed February 29, 1952. The continuously moving sheet of fibrous cellulose xanthate is then compressed into a dense, thin sheet with calendaring rollers. When increased solution of the cellulose xanthate is desired, a suitable solvent can be sprayed onto the sheet just in advance of the rolls. The compressed sheet is then treated with any suitable regenerating liquid such as the usual bath used to regenerate cellophane or a Miiller bath of the type used to regenerate viscose filaments. The regenerating may be done by passing the sheet into a bath of regenerating liquid and then subjecting it to the usual finishing treatment of washing, desulfuring, bleaching, softening and drying.

The process of the inveniton eliminates the following expensive and time consuming operations from the conventional viscose process used in forming sheet cellulose: shredding and aging the alkali cellulose, dissolving the cellulose xanthate to form viscose, filtering the viscose dispersion, ripening the viscose, and casting the viscose through a thin slot.

The product of our invention is a sheet of regenerated cellulose having dispersed therein residual portions of cellulose fibers. The improved regenerated cellulose sheet has relatively high durability due to several factors including: high D. P. (degree of polymerization) of the constituent cellulose due to elimination of alkali cellulose aging; high stability of the sheet toward deformation, shrinkage and thickness changes due to the high cellulose content of the film in its initial regeneration stages; and an interlocking network of residual cellulose fibers within the sheet.

The accompanying drawing illustrates more or less diagrammatically an arrangement of apparatus which may be used in carrying out a process of the invention.

The apparatus shown in the drawings comprises four principal operatively connected sections or units, A for steeping the cellulose to form alkali cellulose, B for treating the alkali cellulose with carbon bisulfide to form cellulose xanthate, C for compressing the fibrous cellulose xanthate to form a dense sheet, D for regenersting the cellulose xanthate, and E apparatus for the finishing treatment of the regenerated cellulose. The cellulose sheet S of more or less indefinite length is passed continuously from the supply roll R through the several units in service and it is to be understood that these units can be closely connected for in-line continuous treatment of the sheet.

Unit A comprises two driven rollers 1 and 2 over which the continuous screen 3 travels by rotation of the rollers. This screen passes over the two vacuum chambers 4 and 5 and supports the sheet S while it is being steeped with sodium hydroxide steeping liquor flowed onto the sheet through the orifices 6. The upper rollers 7 and 8 support the continuous belt 9 which rotates at the same speed and direction as the sheet S and screen 3 and forms a dam on the upper side of the sheet and directly over the vacuum section 5. The steeping liquor is sucked through the sheet by the vacuum applied below and a rapid, uniform and complete conversion to alkali cellulose is effected. The roller 2 has a vacuum section 12 directly under the press roller 13 and the excess caustic liquor is both pressed and sucked out of the sheet at the same place.

The sheet of alkali cellulose then enters the nitrogentreating section of unit B comprising rollers 14 and 15 over which the endless screen 16 travels in the direction of the sheet S. In this section, nitrogen is sucked through the alkali cellulose from the hood 17 by the vacuum in chamber 18 to displace and remove trapped oxygen. From this section the sheet passes through the sealing rollers 22 and 23 and enters the xanthating apparatus 24. Roller 23 is paired with another roller 25 over which the endless screen 26 is rotated in the direction of the sheet. Carbon bisulfide is flowed or sprayed onto the sheet through orifice 27 and is sucked into the sheet by the vacuum in chamber 28. The sheet then passes into heating chamber 29, which chamber can be heated in any suitable way, as the steam or hot water coils. If desired, gaseous carbon bisulfide may be sucked through the sheet from pipe 30 by the suction on pipe 31. The xanthated sheet is supported on an open grid 32 comprising freely rotatable spaced rollers while passing through the chamber 39 to the sealing rollers 34 and 35. The sealing rollers 22, 23, 34 and 35 are preferably power driven, soft rubber rollers which press against the sheet to seal it against loss of carbon bisulfide or entry of air and which aid in pulling the delicate sheet through the operation.

The cellulose xanthate sheet S then enters unit C, first by passing between the calendering rollers and 41 which compress it into a dense, thin sheet. Roller 40 is paired with roller 42 to support and drive the endless belt 43. Rollers 40 and 41 and belt 43 may be formed of stainless steel and may be covered with Teflon, a tetrafiuorethylcne polymer which reduces adherence of the xanthate to the press roll. it it is desired to produce a highly transparent sheet, it is advantageous to increase the water content of the xanthate sheet just prior to pressing, and water or a dilute solution of caustic soda containing, say, about 3% of sodium hydroxide, is sprayed in a carefully regulated amount through nozzle 44 onto the surface of the sheet.

The dense, thin sheet of cellulose xanthate from the calendering operation, with or without associated added water or caustic solution, is passed to the regenerating unit D. It first engages the support and guide roller 45 and then enters the regenerating acid solution in tank 46. The sheet is immersed in the solution by being drawn under guide roller 47 and then passes between the press rollers 48 and 49 which are power driven and not only pull the sheet but squeeze out some of the excess solution. In some cases it may be desirable to carry the xanthate film into the regenerating bath while still being supported by the belt on which it is compressed. In unit D the sheet undergoes coagulation and regeneration and then the regenerated cellulose sheet passes through the separate elements of unit E to receive the finishing treatment. The finishing treatment is conventional in the production of regenerated cellulose sheets by the viscose process and comprises washing in tank 50, desulfuring in tank 51, bleaching in tank 52, softening in tank 53, and drying on the heated rollers 54, 55, 56 and 57. The pairs of rollers 60, 61, 62 and 63 are power driven and draw the regenerated sheet through the various tanks for the finishing treatment.

In carrying out a process of our invention, we may, for example, treat a sheet of refined chemical dissolving pulp supplied in the form of a large roll and preferably in a dry state. The sheet is covered with steeping liquor containing from 12% to 22%, preferably from 16% to 18%, of sodium hydroxide. The entire body of cellulose is uniformly and completely converted to alkali cellulose because it is sucked through the sheet without suffering an appreciable diminution in sodium hydroxide concentration. The excess liquor is sucked out by the vacuum in section 12 while pressure is applied to the opposite side of the sheet by roller 13 which presses the sheet against the screen over roller 2. This aspect of our invention, both apparatus and method, are described more fully in our United States Letters Patent 2,614,102 and may be carried out in accordance therewith.

The oxygen trapped in the pores of the sheet is purged out with nitrogen in passing through hood l7 and chambers 1B and the sheet then enters the xanthating apparatus 24 by passing through the sealing rollers 22 and 23. One of the important features of the xanthating stage of the invention is the rapidity with which the carbon bisulfide reacts with the entire mass of the alkali cellulose. Uniform distribution is accomplished by applying liquid carbon bisulfide in an amount equivalent to from 20% to of the weight of cellulose in the alkali cellulose on one side of the sheet and a vacuum on the opposite side to suck it into the sheet. Heat is applied to the sheet to complete the xanthation reaction in chamber 29. The sheet of cellulose xanthate is passed through the sealing rollers 34 and 35 and out of the xanthating cham' her. The xanthating operation may be carried out in accordance with the method of our said patent application Serial No. 274,208.

The sheet of fibrous cellulose xanthate may be compressed in any suitable way. Advantageoitsly, this may be done in calendering apparatus of the general type illustrated. The sheet may be passed through the rollers 40 and 41 in its completely fibrous state and compressed to a dense and relatively thinner sheet. A portion of the sheet may be dissolved before compression by applying a suitable solvent for the cellulose xanthate such as water or a dilute solution of sodium hydroxide. We prefer to control the addition of solvent so as to obtain a solution which has a relatively high concentration of cellulose. Under optimum conditions for the production of a transparent sheet, we efiect a substantial solution of the cellulose, but at a concentration of from 20% to 35% cellulose. In order to produce a transparent sheet, it is preferred to use a pure and highly reactive dissolvingtype chemical cellulose, mercerize it uniformly with optimum concentration of sodium hydroxide, say, about 18% xanthate it uniformly to a high degree, and then subject it to working action in high-pressure multi-stage calendering. While we have shown but a single stage of compression rolls, it is to be understood that any desired numher of stages may be employed to compress the sheet in successive increments. The steeping according to the method of our patent gives the required uniform mercerization and the xanthating according to our application gives the requisite high degree of uniform xanthation. In certain cases it is beneficial to spray or impregnate the fibrous xanthate sheet just prior to pressing in the calendering rollers with a small amount of sodium hydroxide solution, wetting agent, plasticizer, softening agent, regenerating assistant or water repelling element.

It will be apparent from the foregoing description of our process that the constituent cellulose may have a high degree of polymerization due to the elimination of cellulose aging.

The compressed sheet is then treated with the regenerating bath, for example, the type of bath commonly used to regenerate cellophane in the viscose process. Since the customary filtering with its attendant disadvantages is eliminated, we retain in the sheetthose residual portions of cellulose fibers which are usually removed by filtering. These fibers are dispersed through the regenerated cellulose as an interlocking network which reinforce and strengthen the sheet. The regenerated sheet has high stability toward deformation, shrinkage and thickness changes due to the high concentration of cel lulose in the initial regeneration stages.

The regenerated sheet is subjected to the customary finishing treatment comprising washing with water to remove acid, desulfuring with hot sodium sulfide solution, washing, bleaching and softening. The sheet is then passed over heated drying rollers and is finished for use.

The amount of fibrous residue in the film may vary widely, ranging from 1% or less in a highly transparent sheet to 50% or more in a parchment-like sheet in which xanthation is kept to the minimum that will ensure plasticization and adhesion of the pressed fibers into a cohesive film.

The thickness of the finished film depends on the amount of cellulose in the original web as well as on the extent to which the xanthated web is calendered. As one example, a pulp sheet of 500 basis weight (basic weight is the weight in pounds of 2880 sq. ft.) may be xanthated and calendered to yield films having thickness in the range 0.005 to 0.050 inch, while a 100 B. W. sheet may be calendered to yield films in the range 0.001 to 0.010 inch.

In processing a typical 100 B. W. sheet of cellulose having a thickness of about 0.010 inch, the pressed alkali cellulose contains about 35% cellulose, sodium hydroxide and has an approximate thickness of 0.020 inch. After xanthation with carbon bisulfite equivalent to 35% of the weight of cellulose in the alkali cellulose, the porous =swollen web of xanthated cellulose contains about 30% cellulose and has a thickness of about 0.050 inch. After being compressed in the calender, it has an approximate thickness of 0.005 inch. After regeneration, finishing and drying, the sheet has an approximate thickness of 0.003 inch.

Toughness of the film depends on the type of cellulose used, extent of xanthation, extent of calendering and amount of softener used (c. g. about 5% glycerol). Compared on the basis of equivalent thickness, the strong fibrous films have higher tenacity, lower elongation, higher durability in fiexing and better dimensional stability than cellophane films with more uniform directional strength properties, but with somewhat lower degree of transparency. The fibrous films have higher tenacity, slightly higher elongation, higher durability in flexing and better transparency than parchment sheets.

The product of the invention may be in many sheet forms and have varied uses. In one-embodiment the product resembles, and approaches in transparency, ordinary cellophane. In another embodiment it takes the form of a sheet of translucent parchment-like material which is extremely tough and durable. The improved regenerated sheets of the invention are suitable for multiple wrapping uses. Since the sheets furnish a good base for moisture proofing compositions and are extremely tough and durable, they are particularly suitable for use in packaging frozen foods.

In another embodiment, a web of cheese-cloth, glass fabric or other backing may be fed into the calendering press along with the web of xanthated fibers, being embedded in the under side of the compressed xanthate sheet.

In still another embodiment various materials such as abrasive particles may be applied to the top surface of the xanthate sheet and embedded therein with or without embedded backing.

We claim:

1. The process of forming regenerated cellulosic sheets which comprises subjecting a continuously moving sheet of cellulose pulp to treatment with an aqueous solution of sodium hydroxide to form alkali cellulose in sheet form and then with carbon bisulfide to form cellulose xanthate which in sheet form, compressing the sheet of fibrous cellulose xanthate into a dense sheet, and reacting the dense sheet of cellulose xanthate with an acid regenerating solution to form a sheet consisting largely of regenerated cellulose and having residual cellulose fibrous residues dispersed therein.

2. In the process of claim 1, compressing the sheet of cellulose xanthate over a web of fibrous material and embedding the fibers into a composite sheet and then regenerating cellulose in sheet form from the cellulose xanthate in sheet form.

3. The improved process of forming regenerated cellulosic sheets which comprises subjecting a continuously moving sheet of cellulose pulp to treatment with an aqueous solution of sodium hydroxide to form alkali cellulose in sheet form and then with carbon bisulfide to form cellulose Xanthate while in sheet form, treating the fibrous cellulose xanthate with a relatively small amount of caustic soda solution, compressing the porous cellulose xanthate to a dense sheet, and regenerating the dense sheet of cellulose xanthate with an acid regenerating solution to form a sheet of regenerated cellulose having residual portions of cellulose fibers dispersed therein.

4. The improved process of forming regenerated cellulosic sheets which comprises passing continuously a sheet of cellulose of indefinite length through a series of opera tions including steeping the sheet with caustic soda solution and forming alkali cellulose in sheet form, reacting the alkali cellulose sheet with carbon bisulfide and forming cellulose xanthate in sheet form, and treating the fibrous cellulose xanthate sheet with an aqueous acid regenerating solution to regenerate the cellulose and form a sheet of regenerated cellulose having residual portions of cellulose fibers dispersed therein.

5. In the process of claim 4, applying a dilute caustic soda solution to the cellulose xanthate to dissolve a part of the fibrous cellulose xanthate forming a solution in the sheet having from 9% to 35% of cellulose.

6. in the process of claim 4, forming the alkali cellulose of highly purified chemical dissolving pulp and forming a transparent regenerated cellulose sheet having an interlocking network of residual portions of cellulose fibers therein.

7. In the process of claim 4, forming the alkali cellulose of unbleached paper.

8. In the process of claim 4, forming the alkali cellulose from cotton webs.

9. The improved process of forming cellulosic sheets which comprises passing a sheet of highly reactive dissolving type chemical cellulose of indefinite length continuously through an operation comprising treating the sheet with caustic soda solution to effect a uniform conversion to alkali cellulose in sheet form, treating the alkali cellulose while in sheet form with carbon bisulfide and thus effecting a uniform conversion of the alkali cellulose to cellulose xanthate in sheet form, subjecting the sheet of fibrous cellulose xanthate to a compressing operation to form a dense cellulose xanthate sheet, and applying to the dense cellulose xanthate sheet an aqueous acid regenerating solution to form a sheet of regenerated cellulose having residual portions of cellulose fibers dispersed therein.

10. In the process of claim 9, passing the sheet of alkali cellulose directly and without any appreciable aging to the xanthation treatment, and treating the cellulose xanthate in sheet form with the acid regenerating solution to form a regenerated cellulose sheet having a high degree of polymerization.

ll. In the process of claim 9, applying to the fibrous cellulose xanthate sheet prior to compression a dilute solution of caustic soda so as to partially dissolve the cellulose xanthate, the resulting concentration being from 20% to 35% of cellulose, and then compressing the sheet in a calendering operation.

8 Reierenoee Cited in the file of this patent UNITED STATES PATENTS 520,770 Cross at al June 5, 1894 604,206 Cross et al. May 17, 1898 1,703,961 Schmidt Mar. 5, 1929 1,961,914 Richter et a! June 5, 1934 l,994,396 Kilner Mar. 12, 1935 2,045,345 Dreyfus June 23, 1936 2,413,551 Englund Dec. 31, 1946 FOREIGN PATENTS 535,867 Great Britain Apr. 24, 1941 OTHER REFERENCES Cellulose Chem. (Heuser), published by John Wiley and Sons, Inc., New York, 1947 (pages 364-5 and 373 are relied upon).

Cellulose and Cellulose Derivatives (Ott) Published by lnterscience Publishers. Inc., New York, 1943 (page 838 is relied upon). 

1. THE PROCESS OF FORMING REGENERATED CELLULOSIC SHEETS WHICH COMPRISES SUBJECTING A CONTINUOUSLY MOVING SHEET OF CELLULOSE PULP TO TREATMENT WITH AN AQUEOUS SOLUTION OF SODIUM HYDROXIDE TO FORM ALKALI CELLULOSE IN SHEET FORM AND THEN WITH CARBON BISULFIDE TO FORM CELLUIOSE XANTHATE WHICH IN SHEET FORM, COMPRESSING THE SHEET OF FIBROUS CELLULOSE XANTHATE INTO A DENSE SHEET, AND REACHING THE DENSE SHEET OF CELLULOSE XANTHATE WITH AN ACID REGENERATING SOLUTION TO FORM A SHEET CONSISTING LARGELY OF REGENERATED CELLULOSE AND HAVING RESIDUAL CELLULOSE FIBROUS RESIDUES DISPERSED THEREIN. 